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Terrestrial Subsidies
Terrestrial subsidies is primary production on land (allochthonous) that is transferred to aquatic ecosystems as litter fall or dissolved organic carbon.

Terrestrial subsidies or allochthonous inputs into aquatic environments are a major component of organic carbon budgets for aquatic systems. In many ecosystems autochthonous production of carbon is not enough to support the food web and rely on production being enhanced or subsidized to maintain secondary production. Aquatic ecosystems are generally heterotrophic; respiration exceeds production, suggesting the food web is supported externally. The carbon that enters the aquatic ecosystem gets taken up by micro-organisms like bacteria and algae where the carbon may go up the trophic levels by being consumed. This microbrially-mediated transfer of organic carbon has shown to support food webs in lakes and streams.

Organic carbon inputs into aquatic ecosystems come in multiple forms to be utilized. The two main forms of organic carbon inputs are dissolved organic carbon (DOC) or particulate organic carbon (POC). Particulate organic carbon is living organisms like bacteria, phytoplankton, zooplankton, detrital components derived from living organisms, and litter fall. Dissolved organic carbon is organic carbon that has been broken down, is suspended, and considered soluble in water. Dissolved organic carbon has been shown to stimulate heterotrophic production in aquatic settings and that heterotrophic bacteria can use the allochthonous dissolved organic carbon as a carbon source. Particulate organic carbon also stimulates heterotrophic production which becomes available to bacteria or other micro-organisms through decomposition and other consumers by direct consumption.

Measuring Aquatic-Terrestrial Subsidies
Using stable isotopes of hydrogen is a potential way of measuring the allochthonous and autochthonous energy inputss from aquatic or terrestrial subsidies. This process is is done by first gathering organic matter samples such ass litter fall, algae, bacteria, and tissue samples from larger organisms. The organic samples are then usually dried, ground down to a fine powder, and pyrolyzed at high temperatures to produce H2 and CO gases. The H2 gas is analyzed for stable isotope composition using an isotope-ratio mass spectrometer to determine the amount of energy each sample has. The energy input is determined by the amount of stable hydrogen isotopes present with more equaling higher energy input.

Carbon isotopes are another method used In measuring the energy inputs and sources for aquatic ecosystems. To measure the input from terrestrial to aquatic ecosystems a form of organic carbon with an easily traceable carbon isotope, usually carbon 13 (13C), is added to an aquatic system imitating terrestrial input. The tracer carbon is then allowed to go through the system to be absorbed or taken up by organism. Once the tracer carbon has had time to go through the system samples of water, algae, bacteria, and other organism are taken, measured for how much of the carbon isotope had mad it into them. A food web can then be drawn by tracing what organisms have taken up the tracer carbon and how much. Measuring of the isotopes is done by using an isotope ratio mass spectrometer from dried organic samples.